1. Field of the Invention
The present invention relates to an optical integrated device used in optically reproducing magnetically recorded information from a magneto-optical recording medium.
2. Related Background Art
In order to reduce size and a cost of a reproducing head for a magneto-optical recording medium (hereinafter referred to as a disk), an optical integrated device having a wave guide or other optical elements formed on a substrate may be used.
For example, in an optical integrated device shown in EP 345,232 (hereinafter referred to as prior art), a second wave guide for supplying an illumination light beam and a third wave guide for emitting a reflected light beam are arranged in branches at one end of a first wave guide which functions as a forward path of the illumination light beam to the disk and a return path of the reflected light beam. The reflected light beam is analyzed by a so-called differential method to read and reproduce magnetically recorded information of the disk.
The disk is primarily made of a perpendicularly magnetized film such as GdCo or GdTbFe, and the direction of magnetization of the perpendicularly magnetized film is initially oriented either upward or downward. In the area where information has been recorded, the direction of magnetization is reversed. The information to be recorded is represented by the number and/or length of marks having opposite direction of magnetization.
The recorded information is read by utilizing a phenomenon (magnetic Kerr effect) in which a rotation status of a polarization plane of reflected light when a linearly polarized light is illuminated to the disk changes with the direction of magnetization (upward or downward) of the magnetic film of the disk in the area at which the information has been recorded. For example, assuming that the polarization plane of the reflected light is rotated by .theta..sub.k degrees relative to the polarization plane of the incident light when the direction of magnetization is upward relative to the incident light, it is rotated by -.theta..sub.k degrees when the direction of magnetization is downward relative to the incident light.
In the prior art, the differential method is used to read and reproduce the recorded information or detect the rotation status of the polarization plane due to the Kerr effect.
In the differential method, the reflected light is split (by a TE-TM mode splitter arranged in a wave guide) into two polarized light components having mutually orthogonal polarization planes and substantially equal light intensities, and the split beams are directed to photo-electric converters (detectors) to detect an output difference (a difference between longitudinal components of oscillation) from the detectors. It is advantageous in terms of S/N ratio over a direct method in which a conventional analyzer is used as an optical bulk element to measure only one of the polarized light components.
When the differential method is used, it is necessary to set the direction of polarization of the reflected light to 45 degrees relative to the initial polarization plane of the incident light before it is subjected to the Kerr rotation. Thus, in the prior art, a polarization plane rotating element (mode converter) is arranged in the wave guide (single forward path of the light beam) which supplies the detection light.
In the optical integrated device of the prior art, two optical elements, the mode converter and the beam splitter, are required in addition to the wave guide, and those elements and the arrangement thereof significantly affect a reproducing capability.
Namely, in the prior art, the mode converter is provided in the area of the second wave guide (single forward path of the light beam) which supplies the detection light in order to deflect the initial TE mode light (which has a polarization plane parallel to the plane of substrate) by 45 degrees. More exactly, it is controlled to incline by 45 degrees when it is emitted from the first wave guide.
A problem here is radiation loss at a branch point of the first wave guide, and the second wave guide and the third wave guide. When the light passes therethrough, both the TE mode light and the TM mode light (which has a polarization plane normal to the substrate) are subject to 50% radiation loss. A loss of the incident light may be compensated by increasing a laser power but a loss of the reflected light means reduction of read efficiency of the signal component, which is a significant defect.